This invention relates to pneumatic positioners.
Devices of a similar or related description are disclosed in
U.S. Pat. No. 2,558,506 to D. H. Annin; June 26, 1951; PA1 U.S. Pat. No. 2,830,785 to A. Buri; Apr. 15, 1958; PA1 U.S. Pat. No. 2,880,705 to G. W. Scheider; Apr. 7, 1959; PA1 U.S. Pat. No. 2,867,233 to S. L. Adelson; June 15, 1959; PA1 U.S. Pat. No. 2,879,781 to A. E. Gimson; Mar. 31, 1959; PA1 U.S. Pat. No. 2,911,991 to D. R. Pearl; Nov. 10, 1959; and PA1 U.S. Pat. No. 2,942,581 to D. J. Gattney; June 28, 1960. PA1 U.S. Pat. No. 3,087,468 to B. R. Roberts et al; Apr. 30, 1963.
The closest prior art familiar to me is
It is a general object of this invention to provide an improved pneumatic positioner.
Another object of this invention is to conserve air and to make the positioner less prone to clogging from dirty air. This is achieved by deriving the pilot pressure from the output side rather than the input or air supply side, and thus reducing the pilot pressure to typically 50% of the supply pressure. Thus positioners according to this invention require about 50% of the air flow or bleed of positioners wherein the pilot pressure is equal to the supply pressure. Alternatively, larger restrictions and nozzles can be used in positioners according to the present invention for an air flow equivalent to that of prior art positioners, wherein the pilot pressure is equal to the supply pressure, and this makes positioners according to the present invention less prone to clogging from dirty air.
Another object of this invention is to minimize the effects of overshoots in response to changes in signal pressure. This is achieved by incorporating a negative feedback action between the signal piston under the action of signal pressure, and the nozzle and its orifice wherein pilot pressure prevails. The nozzle is affixed to a nozzle support or piston assembly which moves in direct response to changes in servo pressure, and the nozzle motion is always in the same direction as that of the signal piston, thereby negating the throttling effect of the signal piston on the nozzle.
The embodiment of the invention shown in FIGS. 3 and 4 has the following advantages over prior art.
The amplifier relay AMP and the reversing relay RV, i.e. the two output valves, are coupled pneumatically rather than mechanically such as by lever, linkages, etc. This provides:
(a) greater stability of operation and settings in that small motions of mechanical parts relative to each other resulting from wear, thermal effects, and vibrations do not affect operation or calibration;
(b) less critical alignment of parts required, resulting in lower cost of assembly and greater reliability;
(c) lower sensitivity to changes in temperature and thermal gradients;
(d) greater immunity to vibration and shock.
Still another object of this invention is to achieve a faster response and a better dynamic stability than can be achieved by prior art controls that utilize the output pressure of one valve to control the output pressure of a second valve in an inverse manner, as by means of a reversing relay. The improvement in regard to response and dynamic stability is achieved by means of a novel pneumatic pilot circuit. The pressure in that pilot circuit is a function of the force balance relationship between a feedback spring and a signal-diaphragm-piston assembly. As the pilot pressure changes in the course of position control, the pilot pressure is channeled simultaneously to the power control stages of two inversely operating output valves.
Another object of this invention is to provide a pneumatic positioner having valve seats and valve elements that may be permanently, fixedly and rigidly fastened within each of the housings of the relays forming part of positioner according to this invention rather than requiring means to change the relative position of valve seats and valve elements, or move the same as by screw threads or levers. In positioners according to this invention adjustment of the balance pressure level is made by adjusting the compression of a spring, and unlike other prior art positioners does not involve relative positioning of valve seats or nozzles and valve elements.
Other objects of this invention will become apparent at this description of the invention progresses.